Filtration and membrane processes for liquid phase separations include cross-flow filtration, microfiltration, ultrafiltration, nanofiltration, and reverse osmosis. Each of these processes can have the filtration rate (membrane flux) reduced by concentration polarization, a phenomenon well known in the liquid-phase membrane field. Historically, the deleterious effects of concentration polarization, mainly flux reduction, have been addressed by pumping a liquid feedstock over a filter or membrane surface at a moderate to high cross-flow velocity. For example, in microfiltration and ultrafiltration processes, typical cross-flow velocities can be in the range of from about 2 m/sec to over 5 m/sec.
Gaslift and Slug Flow Membrane Devices. One alternative to using high cross-flow velocity to control membrane flux is to entrain gas bubbles in the liquid being processed in a gaslift operating mode. A review of gaslift operations can be found in the review article of Z. F. Cui, et al. (“The Use of Gas Bubbling to Enhance Membrane Processes”, in J. Mem. Sci. 221 (2003) 1-35). The airlift regime that has been found most beneficial for several membrane configurations is “slug flow”, also called Taylor flow. This is especially useful for a membrane device with a well-defined feed flow path, such as a tubular membrane element with flow inside the tube. This may be contrasted with hollow fiber membrane modules with liquid feed on the fiber exterior. Cui, et al. report that using a slug flow membrane device in downflow is more effective than in upflow. Further, Cui, et al., also determined that the best performance was achieved when the slug flow membrane device was actually situated at a 50° angle of inclination.
Several research organizations have worked with slug flow membrane devices, usually tubular membranes. The membrane materials employed have been both polymeric and ceramic. Tests have included both single tubular elements as well as multi-tubular membrane modules. For example, a research group at INSA, Centre de Bioengenierie, Toulouse, France has published extensively on their work with single ceramic tubular membranes, showing the therapeutic effect of operating in the slug flow regime for membrane flux enhancement (M. Mercier, et al., “How Slug Flow Can Enhance the Ultrafiltration Flux in Mineral Tubular Membranes”, in J. Mem. Sci. 128 (1997) 103-113; M. Mercier, et al., “Yeast Suspension Filtration: Flux Enhancement Using an Upward Gas/Liquid Slug Flow—Application to Continuous Alcoholic Fermentation with Cell Recycle”, in Biotech. and Bioeng., Vol. 58, No. 1, pp. 47-57, Apr. 5, 1998; M. Mercier-Bonin, et al., “Hydrodynamics of Slug Flow Applied to Cross-Flow Filtration in Narrow Tubes”, in AIChE Jl., Vol. 46, No. 3, pp. 476-488, March 2002).
A research group at the Nanjing (China) University of Technology has reported on the highly beneficial effect of slug flow for flux enhancement for a tubular membrane bioreactor and suggested the potential applicability to small diameter multichannel membrane devices (N. Xu, et al., “Design and Application of Airlift Membrane-Bioreactor for Municipal Wastewater Reclamation”, paper presented at The 13th Annual Meeting of the North American Membrane Society (NAMS), May 11-15, 2002, Long Beach, Calif.
Another research group active in the development and evaluation of slug flow tubular membranes is the School of Water Sciences, Cranfield University (UK). Results of testing with multi-tubular polymeric membrane devices have been published (In-Soung Chang and S. J. Judd, “Air Sparging of a Submerged MBR for Municipal Wastewater Treatment” in Process Biochemistry 37 (2002) 915-920). Tests at Cranfield have used polymeric tubular modules from Milleniumpore (UK) and X-Flow (NL).
Results from extensive pilot tests of many polymeric membranes used in airlift membrane bioreactors for sewage treatment, including X-Flow multi-tubular modules, have been published by DHV Water BV (NL). A report is available from IWA Publishing (H. F. van der Roest, et al., “Membrane Bioreactors for Municipal Wastewater Treatment”, IWA Publishing, London, UK (2002)). Results for X-Flow's multi-tubular polymeric membrane modules operated in slug flow are given in pp. 69-83 of this report.
A related patent is U.S. Pat. No. 5,494,577 (Rekers) assigned to Stork Friesland B. V., which discloses an airlift membrane bioreactor that comprises, in part, an airlift multi-tubular membrane device.
All of the above publications show that the use of slug flow involving flowing a mixture of gas and liquid through a membrane device is effective in reducing concentration polarization at the membrane surface during operation, thereby increasing membrane flux. This can be achieved at a liquid cross-flow velocity substantially less than that required to obtain the same membrane flux by solely pumping a liquid feedstock through the membrane device.
Monolith-Based Membrane Devices. Multi-channel monolith membrane devices have been employed in cross-flow membrane separation processes for many years. There are numerous manufacturers of devices, generally of small diameter (<50 cm) and with a relatively few number of channels in the device, typically less than about 50.
Of relevance to the present invention are large-diameter monolith devices that contain one or more “filtrate conduits” within the structure to efficiently extract filtrate from within the structure and convey the filtrate to an external filtrate collection zone. These constructions permit the manufacture of ceramic filtration and membrane modules at substantially reduced costs from those typically available. The monolith-based filter and membrane devices and processes relevant to the present invention, and included herein by reference, include:    1. U.S. Pat. No. 4,781,831 (Goldsmith), which discloses in FIG. 5 therein, and described in the patent Specification, a cluster of individual multiple passageway monoliths arranged to have “filtrate flow conduits” formed by the space among the monolith elements.    2. U.S. Pat. No. 5,009,781 (Goldsmith) and U.S. Pat. No. 5,108,601 (Goldsmith), which disclose therein in the Figures and Specifications unitary monolith structures with filtrate conduits formed within the monoliths.    3. U.S. Pat. No. 6,126,833 (Stobbe, et al.), which discloses structures comprised of an assembly of monolith segments containing both segment internal filtrate conduits and a filtrate conduit arrangement formed by the gap among the monolith segments.    4. U.S. patent application Ser. No. 10/261,107 (Goldsmith) filed Sep. 30, 2002, entitled “Airlift Membrane Device and Membrane Bioreactor and Membrane Bioreactor and Bioreactor Process Containing Same”. This application discloses monolith structures with filtrate conduits used in an airlift membrane bioreactor.    5. U.S. patent application Ser. No. 10/812,538 (Goldsmith), Divisional application to Ser. No. 10/261,107.
The present invention embodies the large diameter monolith membrane module structures with filtrate conduits disclosed in the Goldsmith and Stobbe patents and patent applications, and further incorporates modifications to permit gas-liquid flow through the modules, especially in a slug flow mode.
Monolith Loop Reactors. The use of slug flow (gas/liquid) through multi-channel monolith devices has also been employed for catalytic (non-membrane) loop reactors. A research group at the University of Delft (NL) has published information about gas/liquid downflow monolith loop reactors. The downflow Taylor flow mode developed results in very high mass transfer between the liquid, gas and catalyst surface on the monolith passageway walls. (See: J. J. Heiszwolf, et al., “Hydrodynamic Aspects of the Monolith Loop Reactor”, Chem. Eng. Sci. 56 (2001) 805-812; T. A. Nijhuis, et al., “Monolith Catalysts as Efficient Three-Phase Reactors”, Chem. Eng. Sci. 56 (2001) 823-829; J. Heiszwolf, “The Monolith Loop Reactor an Alternative to Slurry Reactors”, presented at ISCRE 16, Krakow (PO), Sep. 12, 2000). The mass transfer mechanism in the Delft monolith loop reactors is like that in slug flow membrane devices. The Delft studies employ a downflow slug flow device rather than the airlift upflow described previously.
Additionally, use of slug flow in catalytic monolith loop reactors has been disclosed by T. R. Boger in the published U.S. patent application 2002/0081254 A1 (Jun. 27, 2002).
For many years, the effectiveness of slug flow in (a) enhancing performance in membrane devices, especially tubular and multi-tubular configurations, and (b) enhancing mass transfer in monolith catalytic reactors has been recognized. Also, for many years, monolith-based membrane devices have been sold commercially. Nevertheless, no one has recognized that the use of large monolith-based membrane devices in the slug flow regime provides a very attractive operating mode in that membrane system complexity and cost can be significantly reduced. This realization is the basis for the present invention.